class MongoDBRouter:
    """
    A router to control all database operations on models in the
    mongolog application.
    """
    def db_for_read(self, model, **hints):
        if model._meta.app_label == 'mongolog':
            return 'mongodb'
        return None

    def db_for_write(self, model, **hints):
        if model._meta.app_label == 'mongolog':
            return 'mongodb'
        return None

    def allow_relation(self, obj1, obj2, **hints):
        if obj1._meta.app_label == 'mongolog' and \
           obj2._meta.app_label == 'mongolog':
           return True
        return None

    def allow_migrate(self, db, app_label, model_name=None, **hints):
        if app_label == 'mongolog':
            # 确保指定的app只会在指定的db上执行迁移
            return db == 'mongodb'
        elif db == 'mongodb':
            # 确保其他的app不会在指定的db上执行迁移
            return False
        # 其他场景不处理
        return None
